236 THE FLOWER. 



the same part of the proboscis against the low anthers of the 

 short-stamened and the low stigmas of the short-styled form. 



426. Moreover, Dar- 

 win has ascertained by 

 microscopical examina- 

 tion that the pollen of 

 the two differs in size or 

 shape, and by experi- 

 ment that it is less 

 active upon its own 

 stigma than upon the 

 other ; indeed, that in 

 many cases (as in some 

 species of Linum) it is 

 quite inactive or impo- 

 tent not only upon its 

 own stigma but upon its 

 own-form stigma, while 

 it is prepotent on the other, and this reciprocally of the two 

 forms. 1 Here, then, are flowers structurally hermaphrodite, but 

 functionally as if dioecious, securing all the advantages of the 

 latter, along with the economical advantage that both sorts of 

 individual and every blossom may bear seed. With dioecism 

 only about half the plants could be fruitful. 



427. Heterogonous Trimorphism. A threefold heterogonism 

 is known in certain species of a few genera ; and this complica- 

 tion may have certain conceivable advantages over dimorphism. 

 Where seedling dimorphous individuals are few and far between 

 (those multiplying from root would all be alike), there would 

 be an even chance that any two near each other were of the 

 same form and therefore sterile or imperfectly fertile. But if 

 the organization were of three forms, any two of which inter- 

 crossed with perfect fertility, the chances (as Darwin remarks) 

 are two to one that any two plants were of different forms, and 

 therefore by fertilizing each other completely fruitful. 



428. The earliest known instance of three forms as to recip> 

 rocal relative length of stamens and pistil is that of Lythrum 



1 Impotence of own pollen, either absolute or relative, occurs no less in 

 certain flowers which are not dimorphous, as in Corydalis, some species of 

 Passiflora, &c. On the contrary, many dimorphous flowers are in a certain 

 degree self-fertile, especially in the long-stamened and short-styled form. 

 These subjects are physiological, and belong to another volume. 



FIG. 468. Long-styled flower of Pig. 467. laid open. 469. Long-atamened flower of 

 the same laid open. Both equally enlarged. 



